It is known that wind turbines of today comprise a nacelle coupled to the rotor hub and the wind turbine blades in which a generator and a converter are arranged. The generator transforms the mechanical energy from the rotating rotor shaft into an electrical energy which is then transferred to the converter. The converter is coupled to a power grid and converts the power output from the generator into a power output which matches that of the power grid.
Until recently it has been known to use permanent magnet generators (PMGs) in wind turbines which in the recent years have increasing both in size and power output. This means that the generator in the wind turbines also has to increase in size and weight which make the generators more expensive and more difficult to handle during installation. As the power output of the wind turbines increased to over 6 MW, a conventional generator or PMG would no longer be suited for such large wind turbines due to the required size and weight of such generators. Such large and heavy generator would mean that the wind turbine tower would have to be significantly reinforced and/or the generator would have to be installed in sections due to the limited lifting capacity of the crane used to install the wind turbine.
During the recent years, high temperature superconductors have become commercially available allowing the latest generation of wind turbine generators (high temperature superconductive (HTS) generators) to be used in large wind turbines. A person skilled in the art knowing the problems of the PMG and wanting to improve the generator design for large wind turbines would then be motivated to use to a HTS generator instead.
Such a HTS generator allows the current density for the rotor coils to be considerably increased which in turn means that the power density of the generator also increases. This allows the size and weight of the HTS generator to be reduced while maintaining a high power output and also reducing the costs of such large wind turbines. The reliability of the drive train and the service intervals may be further increased by using a direct drive coupling to the rotor hub instead of a gearbox coupling which is sensitive to wind gusts and mechanical stresses in the drive train.
The HTS generator typically comprises three phases inducing a high magnetic field inside the generator due to the high current density in the rotor coils which in turn forms a high electromagnetic torque in the rotor and hence the rotor shaft. GB2416566A discloses such a wind turbine in which the HTS generator is coupled to the rotor hub. The HTS generator is then coupled directly to a converter via a three-phased coupling. The converter comprises a rectifying unit coupled to the generator and a rectifying circuit coupled to the power grid using three phases, wherein a DC link is arranged between the rectifying unit and the inverting circuit. “Superconducting devices in wind farm” by Xiaohang Li discloses a similar wind turbine with a hybrid drive train comprising a simple gear system coupled to a HTS generator. The HTS generator is coupled to a converter via a three-phased coupling which matches the phases of the power grid.
These configurations have the disadvantage that when a failure in the converter is detected, such as a short circuit occurring in at least one of the IGBTs, the wind turbine has to stop the operation of the drive train and the rotation of the wind turbine blades. The short circuit will cause the generator to initiate a brake torque which is nine to ten times greater than the nominal torque of the generator. This may lead to serious damage of the drive train, in particular the generator, the rotor shaft, the rotor hub, and the wind turbine blades, due to extreme decelerations of the rotating parts.
This problem arises when the person skilled in the art wants to improve the generator design for large wind turbine by using a HTS generator, as the PMG does not suffer from the same problem due to the relative low current density occurring in its rotor coils.
US 2009/0295168 A1 discloses a wind turbine comprising a super conducting generator having a stator assembly and a rotor assembly arranged relative to the stator. The stator assembly comprises a plurality of double helix shaped windings that define the stator coils which are arranged relative to each other in series to form six or twelve phases. This document is silent about how the multiple phases are coupled to the rest of the drive train of the wind turbine.
DE 4032492 A1 discloses an electrical machine comprising a converter unit coupled to a generator having at least two sets of stator windings via switching means. The switching means are used to select between different configurations between the sets of stator windings that make up the stator coils. When the switching means are turned off, each set forms at least three phases that are in-phase with the other two sets. When the switching means are activated, the sets are combined to form a hybrid configuration having six or twelve phases. In this configuration, each set of stator coils is coupled to the same converter unit where switches are used to control the number of phases between the generator and the converter unit.